1. Field of the Invention
This invention relates to the lithography technology, and more particularly to a method of defining patterns in a small pitch and a corresponding exposure system that can define patterns in a small pitch.
2. Description of the Related Art
To increase the degree of integration of semiconductor devices, the resolution in lithography processes has to be further enhanced. FIG. 1 shows a typical arrangement between a photomask 100, a lens set 110 and transferred patterns 120 in a lithography process in the prior art, wherein the lens set 110 is represented by only one lens.
The resolution R of the lithography process is defined as “R=k·λ/NA”, wherein k is a constant mainly related to the photomask 100, λ is the wavelength of the exposure light and NA is the numerical aperture of the exposure system. The numerical aperture is defined as “NA=n·sin(θ/2)”, wherein n is the refraction index of the medium in which the lens set 110 is working and θ is the angle of the maximal cone of light that can enter or exit the lens set 110.
Accordingly, to obtain the largest NA or the highest resolution at given k, λ and n values, θ has to be equal to 180°. However, this is impossible for any lens set alone.